Back to HWF Library Page Gossman Consulting, Inc

HWF NOTES©


Gossman Consulting, Inc.                     January, 1994

As many of you are aware, existing EPA SW-846 methods of digestion for cement plant process streams including hazardous waste are inadequate and do not work. Bruce Pederson, Systech Corp., has prepared a microwave digestion technique that does work and will shortly become an ASTM method with the designation D5513. GCI is providing this method here is its entirety. Thanks to Bruce for this important contribution.

Standard Practice for the Microwave Digestion of Industrial Furnace Feedstreams for Trace Element Analysis

1.0 SCOPE

1.1 This practice describes the multi-stage microwave digestion of typical industrial furnace feedstream materials using nitric, hydrofluoric, hydrochloric, and boric acids for the subsequent determination of trace metals.

1.2 This practice has been used successfully on samples of coal, coke, cement raw feed materials, and waste-derived fuels composed primarily of waste paint-related material in preparation for measuring the following trace elements: Ag, As, Ba, Be, Cd, Cr, Hg, Pb, Sb, and Tl. The practice may be applicable to elements not listed above.

1.3 This standard does not purport to address the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific hazard statements are given in Section 7.0.

2.0 REFERENCED DOCUMENTS

2.1 ASTM Standards

D 1193 - Specifications for Reagent Water

3.0 SUMMARY OF PRACTICE

3.1 A weighed portion of the feedstream material is combined with concentrated nitric acid in a Teflon-lined digestion vessel, and heated in a microwave digestion unit. Following a programmed heating cycle, the vessel is vented and specified quantities of hydrofluoric and hydrochloric acids are added, and the mixture undergoes further microwave heating. Following this heating cycle, the vessel is vented and a specified quantity of 4% boric acid solution is added, and the mixture undergoes a final microwave heating. Following this final heating cycle, the vessel is vented, the contents are quantitatively transferred to a volumetric flask and brought to volume. Typically, the only undissolved material is particulate carbon. If particulate matter is observed, filtration or centrifugation may be needed. The digested sample is ready for analysis.

4.0 SIGNIFICANCE AND USE

4.1 The Boiler and Industrial Furnace Regulations require that boilers, cement kilns, and other industrial furnaces utilizing waste-derived fuel adhere to specific guidelines in assessing potential metals emissions. A common approach for estimating potential emissions is performing total metals analysis on all feedstream materials. The practice describes a multi-stage microwave-assisted digestion procedure that solubilizes trace elements for spectroscopic analyses.

5.0 APPARATUS

5.1 Microwave Digestion Unit - equipped with an automatic turntable, pressure controller, and closed PFA-line digestion vessels equipped with pressure relief/rupture membrane fittings or equivalent pressure relief device. The unit should comply with applicable federal and/or state standards for microwave leakage. The user must follow specific manufacture's instruction for system installation.

Note 1: The digestion unit used in developing this practice was equipped with a pressure controller, automatic turntable, exhaust fan, and programming capacity (time, % power output, pressure settings). The unit delivers 630 W of power at 100% output. The lined digestion vessels consist of a high strength polymeric vessel body and cap, inner PFA liner and rupture membrane housing and PFA vent stem. These vessels have a maximum operating pressure of 200 psig. The user must follow specific manufacture's instructions for using digestion vessels.

5.2 Analytical balance - capable of weighing to 0.001 g.

5.3 Labware - high density volumetric polyethylene or polypropylene flasks/sample containers are required for this practice. The user should be mindful of the quality limitations associated with volumetric non-glass labware.

6.0 REAGENTS AND MATERIALS

6.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intend that all reagents conform to the specifications of the Committee of Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.

6.2 Purity of water - Unless otherwise indicated, references to water shall be understood to mean meeting the numerical requirements of Type II water as defined by D 1193.

6.3 Boric acid solution (40 g/L) - Dissolve 40 g of boric acid (H3BO3) in water and dilute to 1 L. It may be necessary to place solution on a combination hot plate/magnetic stirrer and with the aid of a stir bar, allow the solution to mix under gentle heat until boric acid is fully in solution.

6.4 Hydrochloric acid, 37% (d 1.200), HCl

6.5 Hydrofluoric acid, 48% (d 1.150), HF

6.6 Nitric acid, 70%, (d 1.400), HNO3

7.0 HAZARDS

7.1 It is recommended that all operations involving concentrated acids be performed in a laboratory fume hood.

7.2 Hydrochloric acid is a highly corrosive chemical that is reactive with metals and most alkaline chemicals. Impervious gloves and chemical goggles are required for handling. See Material Safety Data Sheet (MSDS) for additional information.

7.3 Hydrochloric acid is a highly corrosive chemical that is reactive with metals and water or steam. Additionally, HF specifically attacks silicate glass making certain fume hood enclosures susceptible to damage. Impervious gloves and chemical goggles are required for handling. See MSDS for additional information.

7.4 Nitric acid is a highly corrosive chemical that is reactive with metals and most organic materials. Impervious gloves and chemical goggles are required for handling. See MSDS for additional information.

7.5 Some samples undergoing microwave digestion can exhibit a rapid pressure rise within the digestion vessel. The potential exists for this type of sample to rupture the rupture membrane and liberate corrosive gases. Because of this, the microwave unit must be vented to a fume hood for proper evacuation of vapors.

8.0 SAMPLE

8.1 Although feedstream materials are generally pulverized powders or liquids, the homogeneity of some feedstream materials can be uncertain. The laboratory sample should be thoroughly mixed or homogenized prior to withdrawing a portion for analysis. This practice assumes that non-liquid feedstreams are pulverized powders at the time of sample preparation.

Note 2: If a non-liquid feedstream material is not in the form of a pulverized powder, it may need to be reduced in particle size to pass through a No. 100 sieve.

9.0 CALIBRATION AND STANDARDIZATION

9.1 Although equipment manufactures specify general power output ratings for microwave digestion units, it is important to verify the actual power output of a specific unit. It is recommended that this Microwave Power Check Procedure be performed monthly.

Power Check Procedure at 100% Instrument Power:

9.1.1 Remove from the instrument cavity the turntable, drive lug, and all vessels.

9.1.2 Adjust the instrument cavity exhaust to minimum air flow (refer to manufacture's instructions).

9.1.3 Program the instrument for 4 minutes time and 100% power.

9.1.4 Transfer 2000 ml ± 2 mL of room temperature (19-25C) water into a 2 L polypropylene beaker.

9.1.5 Measure and record the initial water temperature (Ti) to the nearest 0.1C.

9.1.6 Place the beaker in the right front corner of the instrument cavity (as you face the front of the instrument). This position closely approximates the position of a digestion vessel during processing.

9.1.7 Heat the water for the programmed time.

9.1.8 When the heating cycle is complete, immediately remove the beaker from the cavity, thoroughly stir the water to ensure even heat distribution, and measure the final temperature (Ti) to the nearest 0.1C.

9.1.9 Calculate the delivered power according to the equation:

Power (watts) w/C = T x (35)

  • Where: T = Ti Ti
  • w /C = K x Cp x M /t
  • w = watts
  • K = 4.2 , the factor for converting thermo-chemical calories to watts
  • Cp = 1.0, the heat capacity for water in cal g-1 degree-1
  • M = Mass of water in grams (1 mL H2O = 1 g)
  • t = Time in seconds
  • 9.1.10 If the calculated power is not within the specifications of the unit, do a second test beginning at 9.1.1 for confirmation before contacting the manufacturer.

    10.0 PROCEDURE

    Note 3: Many microwave digestion units are capable of handling up to 12 digestion vessels at a time. The number of vessels processed affects the required digestion times, microwave power absorption, and pressure control settings. For this reason, the procedures detailed below in Section 10.1 and 10.2 are intended for either 4 to 6 digestion vessels or 8 to 10 digestion vessels as is specified in their Microwave Digestion Programs and detailed in Annexes I and II, respectively.

    10.1 Coal, coke and cement raw feed samples are processed as follows:

    10.1.1 Weigh 0.5 g of the feedstream material directly in a PFA digestion vessel liner and record the mass to the nearest 0.01 g.

    10.1.2 Place the liner with sample in the digestion vessel body and with a pipet unit, or equivalent device, slowly add 8 mL HNO3 to the PFA liner. If a chemical reaction is observed, allow the reaction to subside prior to performing Step 10.1.3.

    10.1.3 Place pressure controller cap assembly on the vessel body, and hand tighten to achieve a firm fit. Do not over tighten because this can damage the sealing surface and threads.

    10.1.4 Be sure the pressure relief/rupture membrane screw-type fitting houses a new rupture membrane, and is tightened to finger-tightness. Do no over tighten because this can potentially deform the rupture membrane and damage the threads. If a rupture membrane were to burst during the digestion, the digestion should be stopped and the vessel with the ruptured membrane should be removed from the turntable and the digestion resumed. The sample in the removed vessel should be discarded.

    10.1.5 Place the vessel in the microwave turntable. Insert the pressure sensing tubing from the pressure controller into the pressure controller cap assembly and tighten to finger-tightness. Be sure that the pressure sensing tubing is completely filled with water (from the pressure controller unit to the pressure control vessel prior to inserting this line.

    10.1.6 Repeat Steps 10.1.1 and 10.1.2 for 3 or 5 additional vessels. Cap these vessels with standard vessel cap assemblies. Be sure each vessel cap assembly is tightened to firmness. Evenly distribute the digestion vessels in the turntable to ensure uniform heating of all vessels.

    10.1.7 It is recommended that a blank and a certified reference standard or a matrix spike be processed with each batch of sample of similar composition. A batch is generally considered to be samples that are processed together by the same method, reagents, and the same manipulation. The user will need to determine the number of samples comprising a batch based on the user's quality assurance requirements.

    10.1.8 The samples are ready for microwaving processing. Process the samples by following the Stage 1 microwave program described in Annex I. Be sure the instrument cavity exhaust is set to maximum air flow.

    10.1.9 Following the cooling step (0 power) in Stage 1, the vessels should be cool to the touch. Additional cooling time may be necessary if vessels remain hot. Vent the vessels in a laboratory fume hood or the microwave oven cavity by slowly unscrewing the pressure relief/rupture membrane fitting. The pressure control vessel will need to be vented in the microwave in order to remove the pressure sensing tubing. Remove the vessel cap assemblies.

    10.1.9.1 Flush the pressure sensing tubing with water to remove any trace acid. The flush water should be disposed of as a waste. This is an important step, because damage will occur to the pressure controller if an acidic solution from the pressure sensing tubing is allowed to migrate to the pressure controller.

    10.1.9.2 Using a polyethylene syringe, or other equivalent compatible device, add 4 mL HF, and 2 mL HCl to each vessel. If a chemical reaction is observed, allow the reaction to subside prior to performing Section 10.1.9.3.

    Note 4: The user may choose to vary the amounts of HF and HCl based on prior knowledge of the sample. Samples known to be low in siliceous material may require a lesser amount of HF. In no case should the user reduce the amount of HF or HCl below a 2 mL volume for each acid.

    10.1.9.3 Recap the digestion vessels, making sure the pressure relief/rupture membrane fitting is tightened to finger-tightness. Replace the digestion vessels in the turntable, and reconnect the pressure sensing tubing to the pressure control vessel. Be sure that the pressure sensing tubing is completely filled with water. Place the remaining digestion vessels in the turntable.

    10.1.9.4 The samples are ready for microwaving processing. Process the samples by following the Stage 2 microwave program described in Annex I.

    10.1.9.5 Following the vessel cooling step, vent the vessels as described in Section 10.1.9. Remove the vessel cap assemblies, flush and fill the pressure sensing tubing with water.

    10.1.9.6 Add 35.5 mL H3BO3 solution to each vessel, and recap as described in Section 10.1.9.1.

    10.1.9.7 The samples are ready for final microwaving processing. Process the samples by following the Stage 3 microwave program described in Annex I.

    10.1.9.8 Following the vessel cooling step, vent the vessels as described in Section 10.1.9. Remove the vessel cap assemblies, flush and fill the pressure sensing tubing with water.

    10.1.9.9 Quantitatively transfer digested sample solution to a clean polyethylene 50 mL volumetric flask and bring to volume with water. If any particulate matter is observed, it may be removed by filtration or centrifugation.

    Note 5: Typically, the only undissolved material is particulate carbon, which is readily removed by centrifugation. If filtration is used, the appropriate filter must be cleaned and pre-rinsed with a 1 N nitric acid solution.

    10.1.9.10 Transfer the sample solution to a clean polyethylene sample container.

    10.1.9.11 The sample is now ready for analysis.

    10.2 Waste-derived fuels and other high content samples are processed as follows:

    10.2.1 The procedure for the microwave assisted digestion of waste-derived fuel samples, or other high organics content samples is the same as outlined in Section 10.1, except that the Annex II microwave program should be followed. It should be noted that the programs in Annex II include a cooling step in Stage 2, and a more gradual initial temperature ramp in Stage 1. These steps are important because some waste-derived fuels can be somewhat reactive in the presence of concentrated acids and can exhibit a rapid pressure increase resulting in the venting of the digestion vessel(s). These steps will lessen the likelihood of venting the vessels.

    11.0 PRECISION AND BIAS

    11.1 Precision - No statement is made about the precision since this practice does not produce a specific test result.

    11.2 Bias - No statement is made about the bias since this practice does not produce a specific test result.

    11.3 Appendix I contains analytical results obtained following this procedure on NIST Standard Reference Materials and paraffin oil-based metals standards.

    12.0 KEYWORDS

    Microwave digestion

    Trace elements

    Trace metals

    Coal

    Coke

    Cement

    Waste-derived fuel

    ANNEX I

    (Mandatory Information)

    Digestion Program for Coal, Coke, Raw Feed, and NIST Coal Fly Ash

    For 4 to 6 Vessels

    Digestion

    Step

    Sample Treatment Heating

    Stage Minutes

    Wattage

    ±5w

    Pressure

    Limit psig

    Stage 1 0.5g. sample

    8 mL HNO3

    Vessel Vented

    1. 5

    2. 8

    3. 10

    630

    550

    0

    140

    140

    140

    Stage 2 4 mL HF

    2 mL HCl

    Vessel Vented

    4. 6

    5. 7

    6. 10

    420

    345

    0

    140

    140

    140

    Stage 3 35.5 mL-4%

    H3BO3

    Vessel Vented

    7. 2

    8. 5

    9. 10

    630

    420

    0

    80

    80

    80


    Total Instrument Time: 63 minutes

    For 8 to 10 Vessels:

    Digestion

    Step

    Sample Treatment Heating

    Stage Minutes

    Wattage

    ±5w

    Pressure

    Limit psig

    Stage 1 0.5g. sample

    8 mL HNO3

    Vessel Vented

    1. 7

    2. 10

    3. 10

    630

    550

    0

    140

    140

    140

    Stage 2 4 mL HF

    2 mL HCl

    Vessel Vented

    4. 6

    5. 8

    6. 10

    580

    420

    0

    140

    140

    140

    Stage 3 35.5 mL-4%

    H3BO3

    Vessel Vented

    7. 4

    8. 5

    9. 10

    630

    550

    0

    80

    80

    80


    Total Instrument Time: 70 minutes

    Initial acid concentrations: 11.0% HNO3

    4.0% HF

    4.0% H3BO3

    2.0% HCl

    ANNEX II

    (Mandatory Information)

    Digestion Program for Waste-Derived Fuel, High Organic Content Samples

    For 4 to 6 Vessels

    Digestion

    Step

    Sample Treatment Heating

    Stage Minutes

    Wattage

    ±5w

    Pressure

    Limit psig

    Stage 1 0.5g. sample

    8 mL HNO3

    Vessel Vented

    1. 2

    2. 2

    3. 8

    4. 10

    315

    380

    550

    0

    50

    100

    140

    140

    Stage 2 4 mL HF

    2 mL HCl

    Vessel Vented

    5. 4

    6. 3

    7. 8

    8. 10

    400

    0

    400

    0

    140

    140

    140

    140

    Stage 3 35.5 mL-4%

    H3BO3

    Vessel Vented

    9. 2

    10. 5

    11. 10

    550

    420

    0

    80

    80

    80


    Total Instrument Time: 64 minutes

    For 8 to 10 Vessels:

    Digestion

    Step

    Sample Treatment Heating

    Stage Minutes

    Wattage

    ±5w

    Pressure

    Limit psig

    Stage 1 0.5g. sample

    8 mL HNO3

    Vessel Vented

    1. 4

    2. 4

    3. 10

    4. 10

    380

    500

    565

    0

    50

    100

    140

    140

    Stage 2 4 mL HF

    2 mL HCl

    Vessel Vented

    5. 5

    6. 3

    7. 10

    8. 10

    550

    0

    420

    0

    140

    140

    140

    140

    Stage 3 35.5 mL-4%

    H3BO3

    Vessel Vented

    9. 4

    10. 5

    11. 10

    630

    550

    0

    80

    80

    80


    Total Instrument Time: 75 minutes

    Initial acid concentrations: 11.0% HNO3

    4.0% HF

    4.0% H3BO3

    2.0% HCl


    APPENDIX I

    (Non-mandatory Information)
    Analytical Results of NIST Paraffinic Oil-Based Standards*


    SRM 1632b

    SRM 2704

    Buffalo River



    SRM 1633a

    Oil-based Metals

    Element Bituminous Coal Value

    (Mg/kg)

    Digestion Result

    (mg/kg)

    Percent Recovery

    (%)

    Sediment Value

    (mg/kg)

    Digestion Result

    (mg/kg)

    Percent Recovery

    (%)

    Coal Fly Ash Value

    (mg/kg)

    Digestion Result

    (mg/kg)

    Percent Recovery

    (%)

    Standards Value

    (mg/kg)

    Digestion Result

    (mg/kg)

    Percent Recovery

    (%)

    Ag

    As

    Ba

    Be

    Cd

    Cr

    Hg

    Pb

    Sb

    Tl

    --

    3.72

    67.5

    --

    0.0573

    (11)

    --

    3.67

    (0.24)

    --

    1.6

    4.1

    58

    1.2

    <2.0

    13

    --

    <5.0

    <0.5

    --

    --

    110.2

    85.9

    --

    --

    118.2

    --

    --

    --

    --

    --

    23.4

    414

    --

    3.45

    135

    1.44

    161

    3.79

    1.2

    1.9

    21.0

    319

    1.9

    <2.0

    113

    1.55

    155

    3.7

    1.5

    --

    89.7

    77.1

    --

    --

    83.7

    107.6

    96.3

    97.6

    125.0

    --

    145

    1500

    (12)

    1

    196

    0.16

    72.4

    6.8

    5.7

    1.12

    140.9

    1253

    24.8

    1.1

    182

    0.12

    73

    6.2

    5.9

    --

    97.2

    83.5

    --

    110.0

    92.9

    75.0

    100.8

    91.2

    103.5

    500

    100

    500

    5000

    500

    500

    100

    500

    5000

    --

    484.6

    97.5

    465

    4830

    489

    502

    91.8

    492

    4900

    --

    96.9

    97.5

    93

    96.6

    97.8

    100.4

    91.8

    98.4

    98

    --